Found this online:

According to the Australian Electoral Commission, the decline in voter turnout was the driving force behind the introduction of mandatory voting. It said that voter turnout dropped from 71 per cent in the 1919 election to less than 60 per cent in the 1922 elections.

In order to address the problem, a private member’s bill to amend the Electoral Act was introduced in the Senate in 1924. At the time, it was only the third private member’s bill to be passed into law since 1901.

As a result of the law, the voter turnout at the 1925 election rose to over 91 per cent.

Gradually, states across the country introduced compulsory voting starting from Victoria in 1926, New South Wales and Tasmania in 1928, Western Australia in 1936 and South Australia in 1942.

When enrollment and voting at federal elections was introduced for Australian Aborigines in 1949 it was voluntary, and continued to be so until 1984 when enrollment and voting became compulsory for all eligible electors.